Traditionally, poles are specified on a required strength basis, and pole deflection has not necessarily been a focus of the initial structural analysis. Understanding the important role that deflection plays in utility structure performance, FRP pole manufacturers target certain deflection limits as their goal and inherently exceed the required strength requirements expected.
“A lot of people are under the misunderstanding that composite poles deflect more than wood poles,” Fecht adds. “Although composite materials can have a lower stiffness ratio on a strength equivalent basis, composite manufacturers understand that their products must have comparable deflection performance under load when being installed alongside traditional wood poles, and they design accordingly to achieve this. This is why deflection, and not strength, is typically the governing design characteristic for utilities seeking wood pole equivalency.” And that’s one reason fabricators are encouraging shifts in how utility structures are specified.
As Fecht explains, utility customers typically look for engineered poles to provide “wood pole equivalent” performance. If a utility typically uses an ANSI-designated 50-foot Class 1 wood utility pole, that’s what they would request from a composite pole manufacturer. But Fecht’s response is that what the utility is requesting is a solution as strong as a 50-foot Class 1 wood pole that also deflects in the same range as that specific length and class wood pole. As a result, the term ‘serviceability equivalency’ has emerged to capture both the initial strength requirement and the required deflection performance and is the recommended design approach to use when designing comparable/compatible FRP poles, says Fecht.
“Where it gets interesting is a lot of people don’t understand deflection even on wood poles,” Fecht says. The flex in a wood pole will vary depending on its length and class and also the specific species of wood utilized. Southern yellow pine, western red cedar and Douglas fir, for example, each have a different modulus of elasticity, resulting in different deflection performance. However, reported deflection values for wood poles represent a mean value rather than an absolute, and some in the industry have used these mean wood pole values as absolute value for composite pole deflection. This practice inadvertently results in a stiffer than required, unnecessarily more expensive composite pole to be specified when serviceability equivalency was the initial goal.
The recently released second edition of the American Society of Civil Engineers’ Manual of Practice (MOP) 104, Recommended Practice for Fiber-Reinforced Polymer Products for Overhead Utility Line Structures, addresses this. “The section on deflection, Section 3.10.8 Deflection Serviceability Equivalency, where the term serviceability equivalency has been established in the utility lexicon, has been introduced to more clearly address this specific design approach,” Fecht says.